The invention relates to an integrated flotation filtration plant comprising a flotation basin having side walls and a bottom and having an inlet for supplying a liquid containing suspended matter and an outlet through which purified liquid having a reduced content of said suspended matter is discharged, gas introducing means associated with said inlet and arranged closely above the bottom of said flotation basin to generate a flow of rising gas bubbles for flotation of said suspended matter contained in said liquid entering said flotation basin, comprising further removal means for the flotated mud thus generated and filter means disposed within said flotation basin.
The invention, also, relates to an integrated flotation and filtration process comprising the steps of supplying a liquid containing suspended matter to the inlet of a flotation basin having side walls and a bottom and an outlet, of introducting gas into said liquid while entering said basin to form a rising flow of fine bubbles interacting with said suspended matter and forming a mud floating on the surface of said liquid, of employing removal means to remove the flotated mud from the surface of said liquid, of passing said liquid through filter means to separate any non-flotatable portions of said suspended matter, and of discharging a liquid purified from said suspended matter at said outlet.
In the process of flotation suspended matter in a liquid like water is separated therefrom by generating a rising flow of finely divided gas bubbles within the body of liquid, the matter suspended therein becoming entrained thereby and being caused to float on top of the liquid. The flotated mud thus separated at the surface of the liquid may then become removed by a suitable sweeping device. Alternatively, suspended matter may be generated artificially by adding a flocculent to the liquid in a preconnected flocculation stage in which way the liquid may become purified from suspended matter more effectively. The so-called pressure release flotation has proven particularly advantageous since thereby smaller bubbles can be generated in the liquid as compared to direct gas introduction through nozzles. In the pressure release flotation a portion of the liquid free of any suspended matter may also be circulated; the liquid is saturated with pressurized gas at a pressure in the range of about 3 to 8 bar and, then, introduced in the inlet region into the liquid containing suspended matter through one or more pressure relief valves.
In the mostly rectangular flotation plants the inlet is placed regularly at the bottom of one narrow side of the flotation basin and the outlet is disposed at the opposite narrow side of the flotation basin. Upstream of the outlet removal means including a sweeping device are placed to remove the flotated mud and supply the same to further processing. In many cases deposition of sediments cannot be prevented in the flotation basin so that additional removal means must be provided therefor which convey the sediment accumulated at the bottom of the flotation basin to a channel from which the sediment can be discharged without interference with the flotation process (see Friedrich-Karl Schmidt, "Methods of Flotation", printed in UMWELT, 1975, No. 5, pages 31 to 36).
While the efficiency of known flotation plants is reasonably satisfying, the necessity to use removing apparatus for the flotated mud as well as for deposited sediment represents an undesired and great expense. Also, the known flotation plants do not achieve complete separation of the suspended matter so that in many a case the liquid discharged from the flotation plant still contains a non-negligible concentration of suspended matter. Particularly in the case of variable loads problems will result correctly dimensioning the plant if a breakthrough is to be excluded. It has also been tried to remove such problem by series connecting a filtration plant, however, the expense for the entire plant, then, will become unreasonably large (municipal sewage treatment plant, LUtjensee, Schleswig-Holstein, Federal Republic of Germany).
Another municipal sewage treatment plant (Tangstedt, Schleswig-Holstein, Federal Republic of Germany) has become known in which filter means forming a sand filter bed are arranged in the flotation basin. Such an integrated plant has a problem in that the sand filter bed requires backflushing from time to time for purification which prevents the desired continuous operation of the integrated plant and which requires additionally treatment of the backflush water.